It seems easy to make an ethical argument against punching someone in the face. If you do it, you will physically harm the person. Therefore, you shouldn’t.
But the 18th century philosopher David Hume famously argued that inferences of this type – in which what we ought morally to do (not punch someone) is derived from non-moral states of affairs (punching him will hurt him) – are logically flawed. You cannot, according to Hume, derive an “ought” from an “is,” at least without a supporting “ought” premise. So, deciding that you ought not punch someone because it would harm him presupposes that causing harm is bad or immoral. This presupposition is good enough for most people.
But for Hume and those who subscribe to what is now commonly referred to as the “is-ought gap” or “Hume’s guillotine,” it is not enough.
Hume put the heads of preceding moral philosophers in his proverbial guillotine in Book III, Part I, Section I of his A Treatise of Human Nature. He wrote that every work of moral philosophy he had encountered proceeded from factual, non-moral observations about the world to moral conclusions – those that express what we ought or ought not do. The shift is imperceptible, but it is a significant blunder. “For as this ought, or ought not, expresses some new relation or affirmation, it is necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.”
The blunder, according to Hume, is one of logic. Factual statements are logically different from moral statements, so no factual statements can, by themselves, entail what people morally ought to do. The “ought” statement expresses a new relation, to use Hume’s phrase, that isn’t supported by its purely factual premises. So, a moral judgment that is arrived at by way of facts alone is suspect.
The new, unexplained relation between moral judgments and solely factual premises is characteristic of the broader distinction between facts and values. Moral judgments are value judgments – not factual ones.
In the same way, judgments about the tastiness of particular foods are value judgments. And positive and negative assessments of foods are not logically entailed by just the facts about the foods.
If a cheese enthusiast describes all the facts he knows about cheese – like that it’s made from milk, cultured bacteria, and rennet – that wouldn’t be enough to convince someone that its delicious. Nor would a cheese hater’s listing all the same facts prove cheese is disgusting. Both the cheese lover and the cheese hater make an evaluation that isn’t governed strictly by the logical relations between the facts.
Despite the logical gap between “is” and “ought” statements, and the broader distinction between facts and values, Hume didn’t think moral judgments are hogwash. He just thought they come from sentiments or feelings rather than logical deductions. In Book III, Part I, Section II of the Treatise, he wrote, “Morality … is more properly felt than judged of; though this feeling or sentiment is commonly so soft and gentle, that we are apt to confound it with an idea, according to our common custom of taking all things for the same, which have any near resemblance to each other.”
So, Hume would most likely agree that punching someone in the face is wrong. But he’d say an argument against it is unnecessary, even mistaken. People feel the wrongness. They feel that one ought not punch another in the face – just like a punched person feels the pain.